After almost three decades of AIDS epidemic, we are still challenged with the fact that more than one out of three people with productive infection of HIV-1 develops some form of HIV-1 associated neurocognitive disorder (HAND), which remains refractory to the conventional antiviral therapeutics. In addition, the clinical symptoms of HAND are found to coexist with those of other systemic inflammatory diseases in a significant proportion of patients, some of which are associated with a higher number of activated platelets in the circulation. This may suggest that a common underlying mechanism is at work in these infected individuals. In this respect, HIV-infected patients exhibit increased numbers of activated platelets in the plasma in a manner that relates to the severity of the immunologic deficit, however, it is unknown whether platelets play a pivotal role in the pathogenesis of HAND. Here we demonstrate that the systemic administration of physiologically relevant levels of Tat produces evidence of platelet activation in the plasma, followed by enhanced permeability of the BBB and inflammation in the CNS. These effects were abolished when Tat was administered to animals that had first been depleted of platelets, supporting the notion that platelets might play a crucial role in HAND. Based on these findings, we hypothesize that the activation of peripheral platelets by effector molecules released by HIV-1-infected cells elicits abnormal effects on brain microvascular endothelial cells (BMVEC), thereby altering blood-brain barrier (BBB) integrity and exacerbating inflammation in the CNS. Three fundamentally critical questions are addressed in this proposal. The first is whether inflammatory mediators released by HIV-infected cells also target other immune effector cells, such as platelets (Aim 1). Second, if activated platelets in turn activate BMVEC to stimulate inflammatory vascular changes that are connected with BBB permeability (Aim 2);and last whether the effects caused by platelets facilitate frequent passage of peripheral blood monocytes through an otherwise normal BBB (Aim 3). The obvious corollary to this is whether therapeutic manipulation of platelet activity provides protection from neurologic damages induced by HIV. Thus, the experiments proposed herein have great translational potential for the development of new therapeutic strategies for neuroAIDS. PUBLIC HEALTH RELEVANCE: It is speculated that a common mechanism is engaged by HIV-1 to stimulate inflammatory disorders in various body compartments. This notion will be tested by investigating whether the abnormal activation of blood platelets accounts for the observed neurologic deficits in HIV-infected individuals. The outcome of these studies is expected to further our understanding of disease, which ultimately will lead to new therapeutic strategies.